import pandas as pd
import numpy as np
# pandas中定义两个dataframe如下：
df1 = pd.DataFrame(np.arange(12.).reshape((3, 4)), columns=list('abcd'))
df2 = pd.DataFrame(np.arange(20.).reshape((4, 5)), columns=list('abcde'))
# df1 + df2
# Out[7]:
# a     b     c     d   e
# 0   0.0   2.0   4.0   6.0 NaN
# 1   9.0  11.0  13.0  15.0 NaN
# 2  18.0  20.0  22.0  24.0 NaN
# 3   NaN   NaN   NaN   NaN NaN
#
# 现在要把两个dataframe相加，得到如下结果，该怎么操作？
# a     b     c     d     e
# 0   0.0   2.0   4.0   6.0   4.0
# 1   9.0  11.0  13.0  15.0   9.0
# 2  18.0  20.0  22.0  24.0  14.0
# 3  15.0  16.0  17.0  18.0  19.0
